1. Field of the Invention
The present invention relates to a detector for detecting the presence of hard foreign objects in a material being transported along a feed path. More particularly, the present invention relates to an acoustic detector that is mounted in a fixed position within a rotating feed roll which transports material along a feed path, the detector producing an output signal to stop the feeding of the material when it senses air-borne sound waves caused by impact of a hard foreign object on the feed mechanism which transports the material. The invention is particularly suited for use with agricultural harvesting equipment but finds use in material feed mechanisms of general utility.
2. Description of Prior Art
The problem of hard foreign materials in crop material being processed by agricultural harvesting equipment moving across farm fields is well known and much time and money has been expended in efforts to solve the problem. Magnetic detection systems such as that disclosed in U.S. Pat. No. 3,959,953 to Garrott have been developed for the detection of ferrous metal materials in crop material. However, magnetic detection systems are not capable of detecting non-ferrous hard objects such as stones.
U.S. Pat. No. 4,353,199 to Chow et al. discloses an acoustic system for detecting the presence of stones and other hard foreign objects in crop material being fed through a forage harvester. In this system a piezoelectric acoustic sensor is mounted on the interior surface of a rotatable crop feed roll so as to sense vibrations induced in the feed roll by hard objects striking its exterior surface. However, this system requires that the feed roll be shock mounted so as to isolate the feed roll (and thus the sensor) from machine generated vibrations. Furthermore, since the sensor rotates with the feed roll in which it is mounted, a rotary coupling or slip ring must be provided in order to transmit signals generated by the sensor to circuitry external of the feed roll which controls the stopping of the feed mechanism of the harvester.
The shortcomings of the Chow et al. device are discussed in U.S. Pat. No. 4,720,963 to Weiss et al. As a solution to the problem, the Weiss et al. patent proposes mounting a first feed roll on a movable axis of rotation, and providing an accelerometer to detect translational movement of the first feed roll when a hard foreign object passes between it and a second feed roll which is mounted for rotation about a fixed axis.
This apparatus may perform well in light crop conditions but the foreign objects tend to become embedded in the thick mat resulting from heavier crop conditions with the result that the translational movement becomes insignificant, particularly for objects of smaller size.